Shock acceleration of energetic particles in the heliosphere

The theory of shock acceleration of energetic particles is briefly discussed and reviewed with an emphasis on clarifying the apparent distinction between the V × B and Fermi mechanisms. Attention is restricted to those situations in which the energetic particles do not themselves influence the given shock structure. In particular, application of the theory to the acceleration of energetic particles in corotating interaction regions (CIR) in the solar wind is presented. Here particles are accelerated at the forward and reverse shocks which bound the CIR by being compressed between the shock fronts and magnetic irregularities upstream from the shocks, or by being compressed between upstream irregularities and those downstream from the shocks. Particles also suffer adiabatic deceleration in the expanding solar wind, an effect not included in previous shock models for acceleration in CIRs. The model is able to account for the observed exponential spectra at Earth, the observed behavior of the spectra with radial distance, the observed radial gradients in the intensity, and the observed differences in the intensity and spectra at the forward and reverse shocks.Calculations and resulting energy spectra are also presented for shock acceleration of energetic particles in large solar flare events. Based on the simplifying assumption that the shock evolves as a spherically symmetric Sedov blast wave, the calculation yields the time-integrated spectrum of particles initially injected at the shock which eventually escape ahead of the shock into interplanetary space. The spectra are similar to those observed at Earth. Finally further applications are suggested.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

EXOSAT medium energy observations of Cyg X-2

On July 5.–6. 1983, during the EXOSAT performance verification (PV) and calibration phase, a raster scan of Cygnus X-2 was performed. In contrast to the previously observed smooth intensity variations on timescales of hours, the source revealed a behaviour unknown until now: active periods with high energy flares recurring on time scales of 300–500 s were interrupted by quiet periods of several hours. At all intensity levels the source spectra clearly require a two component continuum (blackbody + thermal bremsstrahlung). In addition, a weak iron emission line with equivalent widths between 39 an 70 eV was detected. The source has a much harder spectrum during the flares than during quiet periods, indicating drastic temperature changes within the emission region, while the absolute iron line flux does not vary. From the spectral characteristics it becomes clear that self-comptonization of the thermal bremsstrahlung spectrum plays an important role. The time variability and spectral behaviour in this peculiar state allow Cyg X-2 to be classified as a Low Mass X-ray Binary System (LMXB) very similar to the prototype of this class, Sco X-1.     

LMC X-4, AO538-66 and surrounding X-ray sources observed with EXOSAT

Repeated observations of LMC X-4 with EXOSAT were carried out in 1983/84 in order to study its 30.5 day cycle and to cover the expected outbursts of the recurrent LMC transient A0538-66. The latter source was inactive during our campaign although a variable circumstellar envelope was still present around the optical counterpart.At least ten further X-ray sources are detected in the CMA field of view around LMC X-4 including the SNR N49 which is the possible site for the March 5, 1979 -ray burster and N63A which appears to be variable in X-ray luminosity. We furthermore discuss the strongest sources that were not present in a previous EINSTEIN survey of the LMC.     

An investigation into the nature of the 4.4 hr periodic source 4U1755-33

We present the results of a continuous 18 hour observation of 4U1755-33 made with the European Space Agency's EXOSAT Observatory. Four 50 min dips in X-ray intensity were observed equally spaced with a period of 4.4 hrs, confirming the periodicity first suggested in White et al. (1984). The dips are spectrally independent. We examine the properties of 4U1755-33 and conclude that the source is most probably point-like and that the metallicity of the absorbing material is at least 600 times less than cosmic values.     

First detection of an X-ray burst and a one hour intensity dip in 4U1323-62

We report the results of a 1.4 10⁴s observation of the region of 4U 1323-62 with the EXOSAT ME. The source has a flux of 7–8 10^-11 erg/cm²s (2–10 keV) and a power-law spectrum with 1.1 < < 1.8. During our observation, the source showed a symmetric 60% dip in its X-ray flux of R~1 hr. The spectrum hardens during the dip. Inside the dip we observed an X-ray burst with a 2–10 keV peak flux of 7 10^-10 erg/cm²s. The burst spectrum is black-body, and shows evidence of cooling during the burst decay. The discovery of a burst from 4U 1323-62 settles the classification of the source; the observation of a dip suggests that we may be able to measure its orbital period in the near future.     

LMC X-4: 13.5 s pulsations and an X-ray flare observed by EXOSAT

EXOSAT observed LMC X-4 on November 17/19, 1983 for one 1.4 day binary period during the high state of the 30.5 day cycle. An eclipse with sharp ingress and slow egress was detected with an eclipse angle of 27.1±1.0 dgr. In the medium energy experiment the source showed a hard power law spectrum. Outside eclipse the source was remarkably constant and only one flare was detected on November 17 at 19 UT lasting for about 1 h. The energy spectrum of the source softens considerably during that time and shows an emission line of cold iron. 13.5 sec pulsations are strongly present during the flare and have also been detected during the quiescent period and during several 1 min flares in another EXOSAT LMC X-4 observation on November 22, 1983. A pulse delay time analysis results in the determination of the pulse period (13.5019±0.0002) s and of the semimajor axis of the orbit of the X-ray star (26.0±0.6) It-sec. These results, together with other available information on LMC X-4, allowed to improve the binary parameters. The mass of the neutron star is found to be 1.34 _±0.44 ^0.48 M_o (95% confidence errors).